U.S. biotechnology firm Moderna Inc. said Thursday its coronavirus vaccine maintained 93 percent efficacy through six months after the second shot, but pointed to the likelihood of the need for a third booster shot in the fall.

"We are pleased that our COVID-19 vaccine is showing durable efficacy of 93 percent through six months, but recognize that the Delta variant is a significant new threat," Moderna CEO Stephane Bancel said in a statement as the company reported its quarterly financial results.

A health worker gives a dose of Moderna COVID-19 vaccine to a medical staff at Wangaya Hospital in Denpasar, Bali, on Aug. 3, 2021. (Anadolu Agency/Getty/Kyodo)

The company attributed the need for a booster shot to what it calls "increased force of infection" resulting from the highly contagious Delta variant that is expected to push up the number of breakthrough infections in vaccinated individuals.

Antibody levels are expected to "continue to wane and eventually impact vaccine efficacy," which leads to the view that a "dose 3 booster will likely be necessary prior to the winter season," the Massachusetts-based firm added.

Moderna's vaccine, which uses a new technology known as messenger RNA, was the second type of two-dose vaccines that were granted emergency use authorization by the U.S. drug regulator in December last year. It is also used in countries including Japan.

The company's latest analysis of a clinical trial showed the vaccine had a 93.2 percent efficacy against COVID-19, the respiratory illness caused by the virus, and 98.2 percent efficacy against severe symptoms, with no deaths reported.

But the data does not include the vaccine's performance against the Delta variant, first detected in India, according to Reuters.